Biometric techniques for determining the identity of individuals are being used increasingly in authentication, recognition, and/or access systems. These techniques use biometric identifiers or human characteristics to verify or identify an individual. The fact that most human characteristics are unique to each individual, are difficult to reproduce by others, and are easily converted to electronic data, is particularly advantageous in biometric identification applications.
Historically, fingerprints have been the most widely used biometric identifiers, as is evident from law enforcement's extensive use of fingerprinting. The recent trends in biometric identification have been toward automating the above-mentioned authentication, recognition, and/or access systems. Most current techniques rely upon correlation methods that use automated detection systems connected to a computer database, for comparing detected biometric data to biometric data stored in the database, to confirm or determine the identity of an individual. Such automated systems have been used to identify individuals before granting access to cars, computers, home or business offices, hotel rooms, and in general, any sensitive or restricted area.
Various optical devices are known which employ prisms upon which a finger whose print is to be identified is placed. For example, the prism has a first surface upon which a finger is placed, a second surface disposed at an acute angle to the first surface through which the fingerprint is viewed and a third illumination surface through which light is directed into the prism. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in the U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example, in U.S. Pat. Nos. 5,109,427 and 5,233,404.
An alternative type of contact imaging device is disclosed in U.S. Pat. No. 4,353,056 in the name of Tsikos issued Oct. 5, 1982, herein incorporated by reference. The imaging device that is described by Tsikos uses a capacitive sensing approach. To this end, the imaging device comprises a two dimensional, row and column, array of capacitors, each comprising a pair of spaced apart electrodes, carried in a sensing member and covered by an insulating film. The sensors rely upon deformation to the sensing member caused by a finger being placed thereon so as to vary locally the spacing between capacitor electrodes, according to the ridge/trough pattern of the fingerprint, and hence, the capacitance of the capacitors.
A further contact imaging device is described in U.S. Pat. No. 5,325,442 in the name of Knapp, issued Jun. 28, 1994, herein incorporated by reference. Knapp discloses a capacitance measuring contact imaging device in the form of a single large active matrix array, formed by the deposition and definition by photolithographic processes of a number of layers on a single large insulating substrate. Electrodes and sets of address conductors formed of metal and field effect transistors are formed as amorphous silicon or polycrystalline silicon thin film transistors (TFTs) using an appropriate substrate of, for example, glass or quartz.
Additionally, a fingerprint sensing device and recognition system that includes an array of closely spaced apart sensing elements, each comprising a sensing electrode and an amplifier circuit, is described in U.S. Pat. No. 5,778,089 in the name of Borza, issued Jul. 7, 1998, herein incorporated by reference.
“Swipe imagers” are also known, wherein an individual places a fingertip into contact with a surface of a contact imaging device and then draws, or “swipes”, the fingertip across a sensing portion of the surface. Images from adjacent portions of the fingerprint are captured and combined in order to construct a composite image of the fingerprint having an area that is greater than the area of a single captured image. In this way, an area of the fingertip that is substantially larger than the sensing portion is imaged. Such an arrangement allows a smaller capacitive fingerprint scanner to be used, which is advantageous due to lower manufacturing costs, improved robustness, and so forth. Also, the small area required is highly advantageous for embedded applications such as with a cell phone, a telephone, a computer (laptop) and so forth.
Methods for processing fingerprint images, for instance to extract data that is useful in correlating an image with previously stored templates, are well known in the art. However, the prior art methods often rely upon the fingerprint image being of a known size and having a known area of interest, and are best suited for use with contact imaging devices that require the individual to hold their fingertip stationary during image acquisition. A particular problem with the swipe imagers described above is that it is unlikely that the individual will “swipe” their fingertip along a perfectly straight line. Accordingly, swipe imagers often produce a fingertip image of arbitrary shape and size. This results in a breakdown of the assumption that are inherent in the prior art processing methods and therefore must be addressed. Typical systems address this issue by increasing the processing to process the entire image in case features are located anywhere therein. Unfortunately, this approach requires changing every biometric identification system for use with each imager.
It would be advantageous to provide an imager that is functional with a plurality of identification processes. It would be further advantageous to provide a method for identifying and/or recognizing individuals based upon a feature extracted from an image of a fingerprint, the image being of arbitrary shape and size.